Batteries Included: A Solar Cell that Stores its Own Power

World’s first “solar battery” runs on light and air

Actually, the patent-pending device invented at The Ohio
State University is both: the world’s first solar battery.

In the October 3, 2014 issue of the journal Nature Communications, the researchers
report that they’ve succeeded in combining a battery and a solar cell into one
hybrid device.

Key to the innovation is a mesh solar panel, which allows
air to enter the battery, and a special process for transferring electrons
between the solar panel and the battery electrode. Inside the device, light and
oxygen enable different parts of the chemical reactions that charge the battery.

Yiying Wu

The university will license the solar battery to industry,
where Yiying Wu, professor of chemistry and biochemistry at Ohio State, says it
will help tame the costs of renewable energy.

“The state of the art is to use a solar panel to capture the
light, and then use a cheap battery to store the energy,” Wu said. “We’ve
integrated both functions into one device. Any time you can do that, you reduce
cost.”

He and his students believe that their device brings down
costs by 25 percent.

The invention also solves a longstanding problem in solar
energy efficiency, by eliminating the loss of electricity that normally occurs when
electrons have to travel between a solar cell and an external battery.
Typically, only 80 percent of electrons emerging from a solar cell make it into
a battery.

With this new design, light is converted to electrons inside
the battery, so nearly 100 percent
of the electrons are saved.

“Basically, it’s a breathing battery,” Wu said. “It breathes
in air when it discharges, and breathes out when it charges.”

For this new study, the researchers wanted to combine a
solar panel with a battery similar to the KAir. The challenge was that solar cells
are normally made of solid semiconductor panels, which would block air from
entering the battery.

Doctoral student Mingzhe Yu designed a permeable mesh solar
panel from titanium gauze, a flexible fabric upon which he grew vertical rods
of titanium dioxide like blades of grass. Air passes freely through the gauze
while the rods capture sunlight.

Normally, connecting a solar cell to a battery would require
the use of four electrodes, the researchers explained. Their hybrid design uses
only three.

The mesh solar panel forms the first electrode. Beneath, the
researchers placed a thin sheet of porous carbon (the second electrode) and a lithium
plate (the third electrode). Between the electrodes, they sandwiched layers of electrolyte
to carry electrons back and forth.

Here’s how the solar battery works: during charging, light
hits the mesh solar panel and creates electrons. Inside the battery, electrons are
involved in the chemical decomposition of lithium peroxide into lithium ions
and oxygen. The oxygen is released into the air, and the lithium ions are
stored in the battery as lithium metal after capturing the electrons.

When the battery discharges, it chemically consumes oxygen
from the air to re-form the lithium peroxide.

An iodide additive in the electrolyte acts as a “shuttle”
that carries electrons, and transports them between the battery electrode and
the mesh solar panel. The use of the additive represents a distinct approach on
improving the battery performance and efficiency, the team said.

The mesh belongs to a class of devices called dye-sensitized
solar cells, because the researchers used a red dye to tune the wavelength of
light it captures.

In tests, they charged and discharged the battery
repeatedly, while doctoral student Lu Ma used X-ray photoelectron spectroscopy
to analyze how well the electrode materials survived—an indication of battery
life.

First they used a ruthenium compound as the red dye, but
since the dye was consumed in the light capture, the battery ran out of dye after
eight hours of charging and discharging—too short a lifetime. So they turned to
a dark red semiconductor that wouldn’t be consumed: hematite, or iron
oxide—more commonly called rust.

Coating the mesh with rust enabled the battery to charge
from sunlight while retaining its red color. Based on early tests, Wu and his
team think that the solar battery’s lifetime will be comparable to rechargeable
batteries already on the market.

The U.S. Department of Energy funds this project, which will
continue as the researchers explore ways to enhance the solar battery’s
performance with new materials.

Related images

Researchers at The Ohio State University have invented a solar battery -- a combination solar cell and battery -- which recharges itself using air and light. The design required a solar panel which captured light, but admitted air to the battery. Here, scanning electron microscope images show the solution: nanometer-sized rods of titanium dioxide (larger image) which cover the surface of a piece of titanium gauze (inset). The holes in the gauze are approximately 200 micrometers across, allowing air to enter the battery while the rods gather light. Image courtesy of Yiying Wu, The Ohio State University. - Download